1. Field of the Invention
The present invention relates to disk drives for computer systems. More particularly, the present invention relates to accounting for thermal disk expansion during a process of writing spiral reference patterns to a disk of a disk drive.
2. Description of the Prior Art and Related Information
When manufacturing a disk drive, servo sectors 20–27 are written to a disk 4 which define a plurality of radially-spaced, concentric data tracks 6 as shown in the prior art disk format of FIG. 1. Each servo sector (e.g., servo sector 24) comprises a preamble 108 for synchronizing gain control and timing recovery, a sync mark 110 for synchronizing to a data field 112 comprising coarse head positioning information such as a track number, and product servo bursts 114 which provide fine head positioning information. During normal operation the product servo bursts 114 are processed by the disk drive in order to maintain a head over a centerline of a target track while writing or reading data. In the past, external servo writers have been used to write the product servo bursts 114 to the disk surface during manufacturing. External servo writers employ extremely accurate head positioning mechanics, such as a laser interferometer, to ensure the product servo bursts 114 are written at the proper radial location from the outer diameter of the disk to the inner diameter of the disk. However, external servo writers are expensive and require a clean room environment so that a head positioning pin can be inserted into the head disk assembly (HDA) without contaminating the disk. Thus, external servo writers have become an expensive bottleneck in the disk drive manufacturing process.
The prior art has suggested various self servo writing methods wherein the internal electronics of the disk drive are used to write the product servo bursts independent of an external servo writer. For example, U.S. Pat. No. 5,668,679 teaches a disk drive which performs a self-servo writing operation by writing a plurality of spiral tracks to the disk which are then processed to write the product servo bursts along a circular path. The spiral tracks are written “open loop” by seeking the head from an outer diameter of the disk to an inner diameter of the disk. The disk drive calibrates acceleration/deceleration impulses to seek the head from the outer to inner diameter in a desired amount of time. Accurate radial positioning of the spiral tracks assumes the calibration process is accurate and that the calibrated acceleration/deceleration impulses will generate a repeatable response over multiple seeks. However, the calibration process will inevitably exhibit some degree of error and the dynamics of the disk drive will change between seeks inducing errors in the radial position of the spiral tracks. Dynamic errors which degrade the spiral tracks written during an open loop seek include vibration of the HDA, flutter and non-repeatable run-out of the disk and spindle bearings, stiction and non-repeatable run-out of the pivot bearings, windage on the head and arm, and flex circuit bias, windage, vibration, and temperature. Errors in writing the spiral tracks will propagate to the product servo bursts, thereby degrading the operating performance of the disk drive and reducing the manufacturing yield. Further, the '679 patent discloses to write the spiral tracks to the disk with a very steep slope over only one or two revolutions which reduces the accuracy of the head position error generated from the spiral tracks. Still further, each spiral track is written to the disk as a high frequency continuous signal (with missing bits), wherein the head position error is generated relative to time shifts in the detected location of the spiral tracks requiring a special timing recovery system as opposed to a conventional servo algorithm.
There is, therefore, a need to improve the servo writing process for a disk drive by reducing the bottleneck and expense of external servo writers while maintaining adequate operating performance and manufacturing yield.